This invention relates generally to a method and apparatus for analyzing a disk drive, and more particularly to a method and apparatus for determining the location and severity of errors on a disk drive or other appropriate recording medium.
Traditionally, in order to determine whether an error is present on a disk drive, it was necessary for an oscilloscope to be connected to the recording medium, and for a signal to be read from the recording medium. A technician manually scrolled through the signal depicted on the oscilloscope for the entire recording medium and visually reviewed the signal, looking for portions of the waveform that did not follow a regular, predictable pattern, to determine if there were any errors. Thus, by way of example, a technician might look at a signal output from a disk drive in order to insure that each of the waveforms has a similar amplitude within a certain tolerance. This method may be most easily used when recording media are encoded using a format known as xe2x80x9cpeak-detectxe2x80x9d in which a technician must confirm that the peaks of all waveforms have consistent amplitude. As is evident, this analysis can be very time consuming since the technician must look at each waveform of the signals reproduced from the entire medium to detect an error. Also it is very difficult for a technician to detect all errors on the recording medium because of the volume of data and subtleness of the error detection.
Recently, drive-encoding format has changed from a peak-detect format to a PRML format. PRML format allows a signal to be checked at various points therealong, and not merely at the peak amplitude position, and thus allows more efficient encoding schemes. In accordance with various PRML formats, there may be anywhere from three to nine or more checkpoints or possible amplitude levels that indicate different data within a signal waveform. With the transition of drive technology from peak-detect to PRML format, interpretation of signal quality through visual analysis of the head signal (i.e., the signal reproduced from the recording medium) has become much more difficult. Whereas peak-detect signals could be analyzed by visual inspection of the location, quality and amplitude of peaks, analysis of PRML signals is much more difficult because of the complexity of PRML wave shapes and because the determination of whether a signal is xe2x80x9cbadxe2x80x9d or xe2x80x9cgoodxe2x80x9d is based on sophisticated processing of the head signal.
For this reason, it is very difficult to manually check a disk drive for errors. Therefore, it would be beneficial to provide an automated disk analysis apparatus and method that determine errors on a disk drive without the need for manual review of the reproduced signal from the entire medium.
Accordingly, it is an object of the invention to provide an improved, automated disk drive failure analysis apparatus and method.
Another object of the invention is to provide an improved automated disk drive analysis apparatus and method which allows a user to automatically analyze a PRML signal received from a disk drive to determine the location of any errors contained therein.
A further object of the invention is to provide an improved automated disk drive analysis apparatus and method in which the PRML signal can be analyzed at any point in the disk drive from the pre-amp stage through output channel.
Yet another object of the invention is to provide an improved automated disk drive analysis apparatus and method which automatically and rapidly finds errors in the head recording signal by determining errors in the Non Return to Zero (NRZ) signal.
A still further object of the invention is to provide an improved automated disk drive analysis apparatus and method which allows a user to view the head signals determined to have errors along with ideal sample values.
Still another object of the invention is to provide an improved automated disk drive analysis apparatus and method which displays the byte offset indicative of the location of a particular error, or allows a user to view a portion of the output head signal at a particular desired byte offset.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and drawings.
Generally speaking, in accordance with the invention, an improved automated disk drive analysis apparatus and method are provided for automatically determining, finding and displaying errors present in the generated head signal of a disk drive. Thus, this improved apparatus and method of the invention improve the user""s ability to:
Analyze PRML signals from the pre-amp through the output channel;
Rapidly find the location of errors on the head or NRZ signals;
View the head signal after equalization (Disk Drive Filter emulation);
Visually compare the equalized head signal to ideal samples values;
Determine the margin available to the Viterbi detector;
View the NRZ data, the corresponding sample values, Viterbi margin, equalized head signal and the direct head signal; and
Analyze the head signal directly.
For the remarkable gains in disk drive capacity to continue, media and head performance improvements are no longer enough. Faced with equally impressive advances in semiconductor technology, disk drive engineers have been working to create a new read-channel architecture that will allow capacity to grow unimpeded.
The answer lies in the construction of the disk itself. The disk""s magnetic poles, with two orientations possible along the track, store the bits as xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d.
When the drive is used in a read operation, the head detects the transition from one pole to anotherxe2x80x94as bit xe2x80x9c1xe2x80x9d to bit xe2x80x9c1xe2x80x9d, for instance. If such transitions are far apart, or low-density, the drive will see isolated pulses. But for greater density, the pulses can either be made shorter, and therefore be placed closer together, or kept wide (longer) but allowed to overlap, and therefore need not return to zero between each pulse.
While the first of these alternatives, represented by Peak Detect systems, is believed to have reached its limits, the second-Partial-Response, Maximum Likelihood (PRML)xe2x80x94is currently seen as the best way to continue to boost capacity. However, additional channel types having superior characteristics, including a number of different algorithms and methods of analyzation, such as Decision Feedback Equalization (DFE), have been proposed, and are currently being developed. While the PRML channel is employed in a preferred embodiment of the invention, the invention is equally applicable to DFE, or any other channel types, which may be employed recording data on a recording medium.
The overlapping pulses of partial-response systems allow much greater density than the shorter isolated pulses of a peak detect system. PRML systems have more samples per xe2x80x9cpw50xe2x80x9d, which is defined as the width of an isolated pulse at 50% of its amplitude. The more complex, or higher-order, the PRML system, the greater the density that can be obtained. As is shown FIG. 1, comparing typical values achieved by available PRML systems with the values achieved by Peak Detect systems, it will be observed that the PRML values allow far denser, and therefore faster and more efficient, encoding schemes, such as E2PR4. PRML encoding schemes may have a density of 2.31 times that of a peak detect encoding scheme. However, the higher-order PRML schemes need very complex circuits and decoders. While the PR4 system works with three vertical levels of samples (encoding levels which must be differentiated), E2PR4 has seven vertical levels which must be differentiated and therefore requires not only a higher resolution of ADC, but a complicated timing and gain recovery circuit and sophisticated Maximum Likelihood detector as well.
Another disadvantage of the more complex PRML schemes is that they are more sensitive to noise.
The process of taking the more-or-less Lorentzian-shaped head response to a magnetic transition and turning it into a correctly shaped pulse is called equalization. This is important due to the need of the Viterbi detector inside the PRML channel chip to receive and analyze correctly shaped pulses. Essentially, equalization is performed in the read channel chip by a Continuous Time Analog Filter (CTAF).
Since a received head signal to be analyzed is typically noisy, and contains pulses which are not quite the desired shape, the DDFA apparatus constructed in accordance with the invention provides an equalization filter to reduce much of the noise and reshape the pulses before it processes and analyzes the waveform. Noise must be eliminated before the sampling occurs, or else it becomes quite difficult to separate noise from the signal desired to be tested. In accordance with a preferred embodiment of the invention, the filter is a digital implementation of a seven-pole, two-zero equiripple filter. This filter may be automatically tuned, or the parameters may each be set manually, as will be described below.
The xe2x80x9cgoodnessxe2x80x9d of a head signal in a PRML drive is ultimately determined by whether the values a Viterbi detector can identify from the head signal through processing are correct. Thus, to test a PRML signal the apparatus must test the data after it has been converted into a digital signal. Since in prior art devices the user typically can only view the preamp head signal on a standard oscilloscope, much of the information the user would require to analyze a PRML signal is not available.
The Viterbi detector is a state machine consisting of two distinct parts, state and transition.
While state is the current magnetization of the disk and some history (memorization of the latest states), transition relates to the change from the current state to the next state. For the Viterbi detector, only two possibilities exist: either the state (medium magnetization) is the same between the current and the next bit periods, or it is not. The detector""s trellis is a mechanism that keeps track of a sequence of magnetization states. The trellis works according to this dichotomyxe2x80x94xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is followed by either a xe2x80x9c0xe2x80x9d or a xe2x80x9c1xe2x80x9d, and so on. When the ML detector makes decisions, the trellis keeps the states of several consecutive time instants and estimates the likelihood of possible xe2x80x9chistoriesxe2x80x9d (higher probability). The higher the order of the PRML system, the larger and more complex the trellis. However, some trellises do not allow certain transitions (d=1 constraint), thereby limiting the extent to which the pulses can overlap.
While a threshold detector, such as the peak detector on a Peak Detect drive, does not use the previous and subsequent samples, a ML (Maximum Likelihood) detector, such as a Viterbi detector, does use historical samples, and thus, by way of example, does not allow xe2x80x9c111xe2x80x9d as a determined sequence, since it is a forbidden sequence of samples. Thus, the detector tries to determine the most probable data pattern for this sequence of samples (21 samples used for PR4).
Proposing several close allowable sequencesxe2x80x94{1 0xe2x88x921 0 1 1 0 0} or {1 0xe2x88x921 0 0 1 1 0} or {1 0 xe2x88x921 0 0 0 1 1}xe2x80x94is easy. But the decision of which is most probable is made based on a sequence of samples, instead of only a single, current sample, and the sequence with the minimum distance (maximum likelihood) is selected as the detection result.
Sequence Amplitude Margin (SAM) is a measure of this minimum distance, and is determined by measuring the error margin or confidence factor of every sample taken by a PRML channel chip by the Viterbi detector and determining an average. Determining that a written bit is either a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9d is the disk drive""s most basic decision. SAM measures the margin by which the Viterbi detector was able to make this decision, the margin or distance being a function of the path metrics and current sample taken together. SAM can provide a prediction of the error rate, and can be used both for characterization and for optimizing equalization. Lower SAM values mean better error rates. Other metrics, such as mean square distance, or the like, which predict error rates in a similar manner may also be used in place of SAM.
The range of SAM values depends on the PRML order (21 for PR4), as does the path metrics, memory or sequence. The range is 0 to xe2x88x921.41 (the negative sign is a convention) for a PR4 channel. The xe2x80x9c0xe2x80x9d in this case signifies that the drive had no margin to make the decision, that it could have read every single bit incorrectly. The xe2x88x921.41 signifies that the drive had as much margin as it could and will never make an error.
FIG. 2 illustrates an exemplary distribution of SAM values taken with three different PR4 drives. The drive represented by curve A to the right in the figure has a distribution of around 0.3 and is a xe2x80x9cbadxe2x80x9d drive. The drive represented by curve B has a distribution of around 0.9 and is an xe2x80x9cOKxe2x80x9d drive. The drive represented by curve C has a distribution of around 1.2 and is a xe2x80x9cgoodxe2x80x9d drive.
The apparatus and method of the invention greatly improve one""s ability to analyze the head signal by utilizing the SAM, in accordance with the results of the Viterbi detector to determine the quality of even the most complex of signals. Thus, by emulating a drive channel to the Viterbi level of operation, the user can now have a PRML signal of any order automatically analyzed in order to more accurately determine if errors exist in the signal. The result of providing these capabilities is that the analysis of PRML signals on oscilloscopes is much easier to interpret, much more accurate, much faster and much more intuitive.
The apparatus and method of the invention also provide users not only the ability to determine the overall quality of a disk drive signal, but also the ability to find the actual location of errors in a signal from a disk drive much more rapidly, and to automatically view these errors. Previously, determining where an error occurred in a PRML head signal was very time consuming, difficult and error prone, requiring scrolling through the signal while manually searching for errors. With the apparatus and method of the invention, finding errors is virtually automatic. In accordance with the invention, five different error finding methods and analysis tools are provided for rapidly finding the location of errors in a disk drive signal. These methods are identified as: Head Analog Compare; ML Distance; ML Compare; NRZ Compare; and Byte Offset. Which method is most appropriate to use depends on the information available to the user.
By definition, a data error occurs in a disk drive when a bit is interpreted as a xe2x80x9c1xe2x80x9d when it should be a xe2x80x9c0xe2x80x9d or vice-versa. It is generally desirable to examine the head signal at the location(s) where data errors occur to gain insight into the cause of the error. The Disk Drive Failure Analysis (DDFA) method and apparatus of the invention provide the tools to help identify the exact or likely location of a data error. In addition, with the channel emulation capability of the DDFA apparatus, the user is able to gain further insight into the cause of an error(s) once its location is determined.
Each of these five Disk Drive Failure Analysis error finding methods analyzes a user selected region of a head signal or an NRZ data signal to determine where errors occur, or are most likely to have occurred (i.e., where the signal was incorrectly interpreted, or where the confidence in the interpretation was low). Once a method is selected, a Find Error function is activated by the user, and the apparatus in accordance with the invention continues to acquire data from the disk drive output signal being tested until one or more errors are found. The user then selects which error he or she wishes to view (if multiple errors are found). As an error is selected to be viewed, the corresponding byte offset location of where the error occurs in the head signal is displayed by the apparatus.
The Head/Analog Compare method compares two signals and identifies where they differ beyond a selected threshold. With this method the user selects a head signal that will be used as the reference signal and stores this reference signal to memory, and then provides an input signal to be tested. The user selects the predetermined maximum allowable difference between the two signals. The apparatus is designed to automatically align the two signals, and identify where a mismatch between the two signals occurs. The apparatus also counts each error and stores its byte-offset location for further review. The errors then may preferably be identified, stored and reviewed in largest to smallest order. As a general-purpose test method, Head/Analog Compare can be applied to finding errors in practically any signal, including VCO synchronization fields, data and servo-information. FIG. 3 depicts the visual output of the invention after performing an exemplary Head/Analog Compare analysis.
The Maximum Likelihood (ML) Distance method acquires a single trace signal from a disk drive and predicts the bit or bits where the disk drive is most likely to create an error. This method uses a full disk drive channel emulation to indicate head signal areas with lower quality. It measures the Sequence Amplitude Margin (SAM), i.e. the distance or margin the Viterbi detector has for making a decision, of all the samples (PRML clock locations). If the SAM is less than a predetermined user-selected value, the head signal location is considered to be an error. The apparatus of the invention emulates a PRML channel and ranks errors by SAM value. A distance or SAM value of xe2x80x9c0xe2x80x9d indicates no margin for a decision and the detector""s lack of certainty as to whether the digital bit should be xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. Preferably, the 100 worst margins on the detector output are displayed along with the SAM value of each. FIG. 4 depicts the visual output of the invention after performing an exemplary ML Distance analysis.
The Maximum Likelihood (ML) Compare method calculates the Viterbi output of a reference signal as well as that of an acquired trace signal, and compares the two to find mismatches, whereas ML Distance (noted above) acquires a single trace and makes an error prediction. Thus, rather than comparing the Viterbi analysis of an input trace signal to a channel emulation, in an ML Compare analysis the results of a Viterbi analysis of the input signal are compared to the results of the Viterbi analysis of the reference signal, and errors are indicated when the results of the Viterbi analyses fail to match. FIG. 5 depicts the visual output of the invention after performing an exemplary ML Compare analysis.
The NRZ Compare method compares the actual NRZ digital data stream received from the disk drive to a reference set of digital data and determines where they disagree. The user can view the data sequences where the digital data from the two sources differ and the apparatus will automatically show the corresponding section on the analog head signal for each error selected. FIG. 11 depicts the visual output of the invention after performing an exemplary NRZ Compare analysis. The apparatus constructed in accordance with the invention further includes a special logic probe for use with the NRZ Compare method. Preferably, the probe may connect as many as 12 NRZ data lines, read clock and control signals to the apparatus of the invention, and stores a non-return to zero reference to which subsequent NRZ acquisitions are compared. Sensing a data error, the probe triggers the oscilloscope to zoom in on the analog waveform corresponding to the location of the data error.
The Byte Offset Position method enables a user to retrieve the byte location of an error and see precisely what kind of error it is, instead of spending time on calculations and delay changes, if the user already knows the location of the error. The user can return repeatedly to find the byte as an offset from the first user data byte after the beginning of the selected data segment.
Thus, in accordance with the invention, a signal from a disk drive may be automatically analyzed by choosing a technique which is consistent with the data which the user is able to obtain. The analysis takes place, and the errors detected by the apparatus are displayed so that the user may review these errors. Thus, in accordance with the apparatus and method of the invention, the detection of an error in a signal output from a disk drive is simplified and is made more accurate.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.